1. Field of the Invention
The present invention relates to a horizontal vibrator assembly and, more particularly, to such a vibrator assembly capable of imparting at least two polarizations of shear wave energy into the earth.
2. Setting of the Invention
In one method of exploring for oil and gas, energy is imparted into the earth and the reflected energy is recorded, processed, and interpreted. Various methods have been used to impart the energy into the earth, including placing a vibrating pad into engagement with the earth's surface, such as a method called "Vibroseis" developed by Continental Oil Company. The Vibroseis-type generator usually comprises a wheeled vehicle, such as a truck, and a hydraulic piston assembly to vibrate the pad vertically to impart compressional (P)wave energy or horizontally to impart a single direction of shear x (S)wave energy.
Geophysicists skilled in the use of shear wave exploration techniques frequently desire to acquire shear waves in at least two polarizations, i.e., in at least two azimuthal directions. Most shear wave generators vibrate the earth's surface at an angle of 90.degree. to the centerline of the wheeled vehicle, and if a different azimuthal angle or polarization of energy is desired the whole vehicle must be repositioned. This repositioning of the vehicle is not only time consuming and expensive, but problems are encountered in positioning the vibrating pad exactly in the same spot as before. If the pad is not positioned exactly the same, errors can be introduced into the recorded signals.
There is a need for a horizontal vibrator assembly that pivots or rotates to impart at least two polarizations of energy. The only known energy vibrator assembly that can produce some shear wave energy in at least two directions is a compressional vibratory assembly described in U.S. Pat. No. 3,866,709, wherein a gyrating hammer generator is used to impart essentially only compressional wave energy in at least two axes as the hammer gyrates. The inventors hereof know of no apparatus for imparting essentially only shear wave energy into the earth in at least two polarizations.